The invention concerns a stent, in particular a coronary stent, comprising at least one first tubular portion and at least one second tubular portion, wherein the first and second tubular portions are connected together by at least one first connecting means.
Stents of that kind are known from the state of the art in many different forms. Those stents are used inter alia in connection with percutaneous transluminal angioplasty (PCTA, Percutaneous Transluminal Balloon Angioplasty), in vascular surgery of the heart. Stents however can also serve to dilate other openings in the body or to keep such openings in a dilated condition. That medical procedure is initially preceded by determining the location of the constriction in a coronary blood vessel. A so-called angioplasty balloon is then moved in the artery which has the constriction, the so-called stenosis, to the location of the stenosis where it is inflated. Due to the radially outwardly directed force of the inflated balloon the constriction is dilated and in the optimum case the original passage cross-section of the previously constricted artery is restored. Besides successful dilation of the vessel however side-effects can also occur, which include local splits in the artery, disintegration effects and projections of plate portions and flakes into the lumen of the artery so that, in spite of the dilation effect, blockage of the vessel can still occur. In addition it is possible that a stenosis can recur due to the vessel wall elastically springing back and/or due to the growth of the intima of the vessel. Statistically, that occurs within six months in the case of over 30% of the patients who were treated with PCTA.
In order now immediately after dilation of the blood vessel to ensure a relatively smooth inside wall surface for the vessel and to be able to avoid renewed stenosis, the stents set forth in the opening part of this specification were developed. Those small tubes serve inter alia in conjunction with PCTA to maintain the vessel flow cross-section which is produced by balloon angioplasty in order thereby to ensure long-term success with the PCTA procedure.
The success of such so-called stenting also depends inter alia on how uniformly the stent can come to bear against the wall of the vessel. For, the more uniformly the wall of the vessel is supported, the correspondingly more probable it is that vessel constrictions will not recur in the region of the stent. In that respect a regular stent structure produces a relatively smooth inside surface for the vessel and, with a relatively smooth inside vessel surface, blood particles can only be deposited thereon with difficulty. In addition growths of the intima into the interior of the vessel are prevented to a greater degree by a regular stent structure which covers over the inside surface of the vessel in a relatively closed configuration.
Stents of that kind with a so-called closed structure are also known from the state of the art. By way of example reference may be made here to one of the best-known stents of that kind, the so-called wall stent. That is known for example from U.S. Pat. No. 4,65,771. This stent which has a closed structure is formed from two wires which are regularly knitted in a mesh-like structure and which extend in a spiral configuration on the longitudinal axis of the stent.
The advantage of the closed structure of stents of that kind is however only achieved at the cost of the disadvantage that the stents involve relative longitudinal stiffness during insertion. Those stents do not therefore make it possible in the optimum manner for the stent to be guided through possibly very severely curved vessel portions in the coronary arteries upon insertion in a direction towards the stenosis to be treated. Those longitudinally stiff stents also cannot be used in the region of curvy vessel portions. In order to avoid those disadvantages of a closed structure, stents have now been developed which are of a so-called modular nature. In the case of those stents of a modular nature, individual portions which are provided with a closed structure are connected together by flexible connections. Stents of that kind are known for example from U.S. Pat. No. 5,104,404.
A disadvantage with the known modular or segmented stents however is that upon crimping of the stent (the term crimping is used to denote mounting the stent on the balloon catheter in a non-displaceable fashion by applying a gentle, radially inwardly directed pressure thereto), the stent may involve a non-uniform behavior. For, in the crimping operation the tubular portions exhibited a different behavior from the individual bars which serve as connecting means, between the tubular portions. In addition there is the disadvantage with the known stents for precisely those reasons that the stent also entails a non-uniform expansion characteristic. With that non-uniform expansion of the known stents the stent may involve inter alia a radial spreading movement of individual bars. That spreading movement is unwanted however as it interrupts the uniformity, which has already been referred to above, of the inside wall of the expanded stent. In addition that can also involve injury and damage to the intima of the stented vessel. In addition, in the case of conventional stents which are composed of various segments, expansion of the stent can involve the stent expanding in a shape referred to as a xe2x80x9cdog-bonexe2x80x9d shape. In that situation, the tubular portions which are at the ends of the stent are expanded to a greater degree than the tubular portions which are more in the central region of the stent. That is also disadvantageous for the above-discussed reasons, in particular in regard to durably suppressing further deposits on the inside wall of the stent.
Therefore the object of the present invention is to further develop a stent of the kind set forth in the opening part of this specification, in such a way that the above-mentioned disadvantages are avoided, and that there is made available a stent in which the flexural flexibility of segmented stents or stents which are also referred to as modular is maintained while at the same time the longitudinal stability of the stent is increased.
In a stent of the kind set forth in the opening part of this specification that object is attained in that the at least one first connecting means is in the form of a double bar.
By means of the double bar according to the invention the invention achieves a longitudinal stability for the stent which is markedly increased in comparison with conventional segmented stents while at the same time the flexural flexibility of the modular design of the stent is maintained. In that way by virtue of the invention it is possible for the stent not to be compressed during balloon dilation. In addition, because of the invention, the adjacent tubular portions cannot come into unwanted contact with each other. In that way, by means of the invention, adjacent tubular portions are prevented from influencing each other upon expansion of the stent in the balloon dilation procedure. In addition, the double bars according to the invention, in contrast to the single bars known from the state of the art, provide as a connecting means between the adjacent tubular portions for a more regular and uniform crimping and expansion behavior on the part of the stent. In that case the invention advantageously avoids inter alia radial spreading of bars upon balloon dilation and also prevents the tubular portions at the ends of the stents from expanding in a xe2x80x9cdog-bonexe2x80x9d configuration.
A particularly preferred embodiment of the invention is distinguished in that the tubular portions each comprise a plurality of cells, wherein the double bar according to the invention connects together cells, which are adjacent in the longitudinal direction of the stent, of the respective portions. This cell-type structure of the tubular portions provides for an optimum expansion behavior on the part of the stent upon expansion thereof.
In a further preferred embodiment the portions are formed from identical cells which are arranged in adjacent relationship in the peripheral direction. That advantageously affords a crimping and expansion behavior on the part of the stent, which is uniform and regular in the peripheral direction.
In a further preferred embodiment the cells of adjacent portions are of identical structure. In that case, it is preferred if all cells of all portions are of identical structure, in which respect it is particularly further preferred if all portions are also of identical structure. In this embodiment, that affords a completely uniform and regular crimping and expansion behavior on the part of the stent according to the invention over the entire length thereof.
In a particularly preferred embodiment of the invention the cells of the portions are each constructed from 2 mutually facing closed bars which extend in a meander configuration in the peripheral direction of the portions and which are connected together to form the cells by means of second connecting means which are in the form of single bars. Closed bars of that kind implement the object according to the invention of maintaining flexural flexibility with at the same time longitudinal stability of the stent in a particularly advantageous fashion as, in this embodiment, the double bars according to the invention can extend between the meander-shaped bars insofar as they engage the upper or lower apex points of the bar meander which extends in the peripheral direction, depending on the demands on the stent. At the same time optimally uniform expansion of the stent becomes possible by virtue of the meander-shaped boundary of the tubular portions.
In a further preferred embodiment of the invention the meander bars are arranged in mirror image-symmetrical relationship on the periphery. In that way in particular the arcuate portions of the meander-shaped bars, which extend outwardly with respect to the mirror-image plane, are used to form the cells of the tubular portions.
In that case it is further preferred if the meander shape of the meander bars substantially corresponds to the configuration of a sine curve along a peripheral line of the tubular portions. That ensures a particularly harmonic expansion behavior on the part of the stent according to the invention as a bar which is curved in a sinusoidal configuration expands in a particularly uniform manner upon expansion of the stent.
In a further preferred embodiment of the invention each second arcuate portion of the bars which are of a sinusoidal meander configuration in the peripheral direction of the tubular portions is flatter than the remaining arcuate portions. That arrangement provides that the cells of the tubular portions, which are enclosed by the meander bars, are also alternately of a varying size. This embodiment provides that the stent according to the invention, even when being introduced into curved or bent vessels, enjoys a sufficiently large free motion between the tubular portions. That free motion or freedom of movement between the tubular portions which is afforded by virtue of the flatter arcuate portions of the bars thus ensures that adjacent tubular portions do not hinder each other or hook one into the other upon bending of the stent. It is further preferred in this embodiment if the double bar or bars connects or connect the tubular portions at locations at which the flatter arcuate portions of the meander bars are provided. In that way upon bending of the stent the stent enjoys a particularly great free motion between the tubular portions so that the above-mentioned advantages are still further enhanced.
In a further particularly preferred embodiment of the invention the apex points of the substantially sinusoidal arcuate portions of the meander bars are of a flattened configuration in the longitudinal direction of the tubular portions. In this embodiment therefore the reversal points of the meanders are provided with double radii. In this embodiment that advantageously ensures that the arcuate portions of the meander configurations do not spread apart in a xe2x80x9cdonkey""s earxe2x80x9d fashion. That in turn advantageously prevents the intima of the vessel suffering from damage and injury which is linked to such a spreading movement.
In that respect this embodiment also generally provides for a more regular crimping and expansion characteristic on the part of the stent. Furthermore, the mechanical loadings on a coating which is preferably provided on the stent surface, that is to say on the bars, are reduced by virtue of the flattened apex points or by the double radii of the meander reversal points. For, due to the double radii involved, upon expansion of the stent the loading on the coating or the loading on the adhesion of the coating to the bar surface is exerted not just on a reversal point in the meander configuration but on the two portions, formed by the double radii, of the reversal or apex points of the meander configurations. Therefore that distribution of the deformation of the meander configurations upon expansion of the stent over the two radius portions of the arcuate configurations of the meanders overall advantageously reduces the loading on the coating on a stent.
A further preferred embodiment of the invention is distinguished in that the second connecting means respectively connect regions of the meander bars which are arranged in adjacent relationship in the longitudinal direction of the tubular portions. In that case it is further preferred if the second connecting means connect two meander bars respectively in the proximity of their regions which are at the maximum longitudinal spacing. In that arrangement the second connecting means which are in the form of single bars preferably extend substantially in the longitudinal direction of the tubular portions. Thus, this embodiment advantageously provides for a minimization in the contraction in respect of length of the stent upon expansion thereof. Those longitudinal bars also serve for lengthwise stabilization of the stent. In that respect those longitudinal bars advantageously co-operate with the double bars according to the invention in regard to lengthwise stabilization of the stent.
In a further particularly preferred embodiment the ends of the second connecting means which are in the form of single bars extend substantially in the peripheral direction of the tubular portions. In this case, in a further preferred embodiment, alternatively thereto or in addition thereto, the single bars may taper towards their respective connecting point to the meander bars.
Then in that way the single bars which serve as the second connecting means form yield hinges or pivots between the meander bars. Those yield hinges advantageously ensure that the radial expansion characteristic of the stent upon expansion thereof is influenced to the minimum possible degree.
In a further advantageous development of the present invention the ends of the double bars are mounted to locations on the meander bars, in the immediate proximity of which the single bar of the second connecting means, which is longitudinally adjacent along the respective tubular portion, is also fixed. In this embodiment, that provides for maximum possible longitudinal stability of the stent, insofar as both bars which extend in the longitudinal direction of the stent, that is to say the double bar and the single bar, extend substantially in mutually aligned relationship in the longitudinal direction.
Further preferred embodiments of the invention are set forth in the appendant claims.